


Lab Report

by whiterose123



Series: Undertail - do you wanna have a good time? [1]
Category: Undertale (Video Game)
Genre: Biting, Dirty Talk, F/M, Gags, Human fetish, Interspecies, Light BDSM, Monster Fetish, Oral Sex, Rough Sex, Shameless Smut, Smut, Socially Awkward Reader, University AU, Vaginal Sex, enemies to lovers?
Language: English
Status: Completed
Published: 2019-11-21
Updated: 2019-11-21
Packaged: 2021-02-18 06:31:01
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,223
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21506683
Author URL: https://archiveofourown.org/users/whiterose123/pseuds/whiterose123
Summary: After a misplaced comment made in a fit of embarrassment, you thought that he hated you.You certainly didn’t expect him to find out that you liked him in the worst possible way.…University AU (Monsters on Surface). Oneshot. Smut with a small amount of plot.
Relationships: Sans (Undertale)/Reader
Series: Undertail - do you wanna have a good time? [1]
Series URL: https://archiveofourown.org/series/1717198
Comments: 26
Kudos: 345





	Lab Report

**Author's Note:**

> Hello! Yes, this is porn! I hope you enjoy!

_He’s so unique…_

Those thick, ivory bones. That fascinating buzz of magic that electrified your nerves by nothing other than being in his presence. God, monsters were so cool. And he... he was extra special.

You were crushing hard…

_So embarrassing…_

What would your friends think? What would your family think if they knew you were infatuated with a monster?

One you didn’t even know the name of, no less…

…

What would _he_ think if he knew a _human_ wanted to jump his bones?

…

The skeleton monster… you remembered the day you first saw him one year prior. It was in your Physics 401 class; he sat two rows in front of you amongst a few of his monster friends. He was one of those guys that everyone seemed to like, a magnetic personality that resulted in him always being surrounded by people.

And you… you were captivated from the get go.

You never could stop staring.

But then you just had to fuck it up.

…

**_Two Months Earlier_ **

_After finishing your lunch, you gathered your things together, haphazardly tossing your notebook in your bag and throwing your laptop back in its case. Slinging the bag around your shoulder, you began to rush to your next class._

_You checked your watch._

**_Shit._ ** _You only had five minutes to get there and it was across campus._

_Hurry. Hurry. Hurry._

_“hey, pal.”_

_You froze upon hearing the voice behind you. It was deep and gravelly... one you would recognize anywhere._

_You slowly turned, coming face to face with… him. Oh god, he looked even more handsome close up… those smooth bones, those delicate eye lights that you could get lost in._

_Heat began to rise to your face._

_He grinned, cocking his head and holding out your laptop case to you. “think ya forgot this.”_

_Oh, that’s so sweet of him… not only was he handsome, but he’s also kind…_

_“luckily it was just within my **radius.** ” He winked. “so i spotted it.” _

_Was that...? Aww, he had a sense of humor too..._

_He’s still looking at you! Oh god…_

_Your hands began to shake, sweat beading at your back._

_Oh god, what should you say? What should you do?! Oh god, oh god, oh god. You couldn't let him find out that you liked him! That would be so embarrassing!_

_Your cheeks burned hotter and you were terrified that you looked like a tomato._

**_Oh no!_ **

_“Don’t touch my things! Monster scum!” you snapped, snatching the laptop and storming away; leaving the bewildered skeleton behind._

_…_

Everything changed after that.

It went from him not knowing you existed to him glaring at you whenever you were within his vicinity. Each day you walked into your shared class with him, he would instantly take note of your arrival, his expression darkening as you shyly hurried to your seat.

It felt like he was going to burn a hole through your head, the way he looked at you…

You couldn’t avoid him either, as he was also in your Physics 402 class the very next semester. A class that you couldn’t drop – you needed to take it this semester to graduate on time.

It was… unfortunate. And each time you saw him, you wished that you could just melt into the ground and disappear.

…You at the very least finally learned his name after seeing it on the sign in sheet…

It was Sans.

…

…

You completely fucked it up.

Why did you say that back then? How could you be so _stupid_?

If only he knew what you were _actually_ thinking… he’d know you were the furthest thing from a racist. You wished you had the courage to tell him. 

But it was completely moot now. There’s no way he would ever want anything to do with you after what you said.

But then…

“As you know, lab starts this week,” the Professor announced at the end of class, “hopefully you know by now which timeslot you’re in. You have all been assigned your lab partner for the semester already – you can check online to see who you’re with. If there are any issues, speak with your TA.”

When you checked online, you spotted… a familiar name paired with yours. 

**Wednesday 3:00 pm Physics 402 Lab. TA: Jordan Brown.**

**Lindsay Hall and Michael Long**

**Kelly Phelps and Michelle Hulbert**

**…**

**…**

**Sans Serif…**

… Partnered with… you…

_Fuck._

The moment you saw it, you quite nearly impulsively emailed your Professor to request a lab timeslot change. But… none of the other times worked with your class schedule…

Maybe he would request a change… there’s _no way_ he would want to be your partner. He obviously hated you.

But yet, when Wednesday came around and you strolled into the lab, you immediately spotted Sans lounging in the chair at your assigned bench space.

Face paling, you almost ran out of the room; _almost_ decided that you didn’t actually _need_ college, that you could survive without a degree, that maybe it’s just best that you moved as far away from him as possible.

But you steeled yourself, taking a deep breath in vain effort to calm your pounding heart as you slowly made your way over to the desk...

... And immediately found yourself reconsidering just leaving the country when his gaze found you, sockets darkening into a glare so mean that if looks could kill…

… Well you’d simply be dead on the floor.

“H-hello,” you stammered as you took your seat next to him.

He snorted in response, crossing his arms and looking away from you.

You shrunk back into your chair, scooting away from him while you made an effort to focus on the TA while she explained today’s experiment and the expectations for lab. And as though mistress fate was trying to play a cruel prank on you, it turned out that lab reports were not written separately in this class but rather expected to be to be completed _with_ your lab partner.

Perhaps you should just move to another planet at this point.

…

“welp, human…”

You were jarred out of your worried thoughts when Sans finally spoke… oh lord, that’s the first thing he has said to you since _the incident._

Fuck, why did his voice have to be so sexy.

“… better get to work.”

Mouth falling agape, you flashed him an incredulous look as he leaned back in his chair, sockets shutting as he made it clear that he intended to put _zero_ effort forth to help you.

“B-but… b-but… I need help, we’ll be here all day if I have to do this alone…”

Opening a single socket to look at you, he again snorted. “you should move faster then.”

“besides…” he said as he stretched his arms, making a popping sound as he folded them behind his skull, again shutting his eyes. “you’ve made it _very_ clear that you don’t want ‘monster scum’ touching things you do.”

“i’d be just a jerk to disrespect that.”

”B-but I never... I never...” Your throat locked up before you could get the words out.

He made a clicking sound, not even opening his eyes. “don’t wanna hear it... you’re wasting precious time, better get working.”

_Ugh…_

At least he was actually talking to you now.

…

Thanks to Sans remaining stalwart in his decision not to help with the project, you had spent the entire three hours of the lab frantically attempting to complete the experiment on time – leaving you with zero time to actually write the report at the end.

Surprisingly enough, he wanted to help with the report since, in his words, he didn’t want a ‘dense human like you’ ruining his grade.

In fact, he didn’t want your help at all with the report as he sat on the floor of your dorm room, rifling through _your_ notes and typing on his laptop all the while.

All you were able to do was sit on your bed and _stare at him_ \- not that you minded that part. And you counted it as progress that he at least wasn’t glaring at you anymore!

Now that you had him alone in a room with you… you wanted to apologize… but you found yourself unable to find the words, throat clamping up in embarrassment every time you made move to attempt it.

You swallowed, a light blush rising to your cheeks. “You sure you don’t want… help?”

“nope.”

“But I-“

“i said no, human. you’ll just mess it up. i saw what a klutz you were in the lab today... can't trust ya with anything,” he stated as though it was the most obvious thing in the world.

Jeez, a little harsh, but you supposed you deserved it.

He peered up at you, before speaking wryly, "must be just terrible for you that i'm touching your stuff."

_Okay..._

You exhaled jaggedly. “N-no, it's not... okay, f-fine... I’m just gonna go use the bathroom then.”

This was going too far. You had to fix this somehow. You were just going to go use the bathroom, regain your composure, and then come back and apologize for what you said back then. Maybe he'd forgive you if you phrased it properly.

Without another word, you exited the room.

One of the things you hated most about living in the dorms was that the bathrooms were communal. You had to walk all the way to the end of the hall to reach the one for your floor, and it was almost always filthy. You certainly wouldn’t miss that when you were done with college. Nontheless, you spent a large amount of time hiding in the stall, making a vain effort to collect your thoughts before finally summoning the courage to return and face Sans.

You raced back down the hall, re-entering your room and expecting to find Sans still typing on the floor…

_What the fuck?! Is that my diary?!_

Yep, there he was, lounging on your bed like it was his own, a stupid grin plastered on his face as he read from _your_ diary.

His eye lights flashed to you and his grin stretched. “some pretty interesting stuff in here i gotta say.”

Face paling in horror, you nearly shrieked, “What are you _doing?!_ That’s private!”

You dashed towards him, intending to snatch it out of his hands… only for him to vanish from existence the moment you were within reach.

You yelped when he re-appeared behind you moments later, waggling the notebook tauntingly in front of you. “yeah, some _real_ interesting stuff…” He kept holding it out of reach as you tried to jump for it, before popping it open and mimicking your voice, “ ‘i saw him in the hallway again today… he’s just so gorgeous… i wonder what he looks like under that blue coat. oh sweet baby jesus, the things i’d do to him if i could.’ “

He laughed before chucking the diary to the side. Your face burned like it was on fire and you found yourself unable to do much else other than just stand there and pray that you would suddenly acquire invisibility as a superpower.

“ya know… i actually thought you hated monsters…”

Your gaze snapped back to Sans, noticing a distinct shift in his tone… but you couldn’t quite place what. You froze, face flushed and breaths uneven, your mind failing to function properly.

He advanced on you, backing you against the wall. Heart pounding, you fritted your hands around, searching for something to grab onto. His gaze was sharp, _intense;_ like he had become a completely different person.

“… but i see now that’s hardly the case.”

His nasal cavity was level with your nose, ivory features stiff and heated breath wafting over your lips…

It was as though time itself had stopped.

_He’s so close…_

You didn’t even think as you clutched his jaw and mashed your mouth against his teeth.

Sans immediately returned the kiss, grabbing your shoulders and slamming you against the wall. You felt an odd buzz wash over your lips and strangely enough, it no longer felt like you were kissing bone but rather actual, warm lips. Magical lips perhaps.

Something hot and gelatinous prodded at you, and you opened your mouth to meet what you presumed was his tongue. Wow, you didn’t even know he had a tongue! Long and oozing the same tingly magic that his ‘lips’ did, it slithered into your mouth and stroked against your own tongue.

Threading his fingers through your hair, Sans tilted your head to deepen the kiss, allowing you little time to adjust as he fervently worked his mouth against yours. It electrified your body, your legs growing numb and knees beginning to quake. Head swimming, you offered no protest when he abruptly broke away and pushed you towards the bed.

Shoving you onto the mattress, he was on top of you in the blink of an eye. He kissed you again, hands snaking under your shirt and tugging at your clothes.

You went along with it seamlessly, more eager than ever, allowing him to pull your shirt over your head before trailing your fingers down the ridges of his cervical vertebrae and attempting to slip his coat off.

You jumped in startle when he growled at you, him snatching your wrists and pinning them against the mattress. “did i _say_ you could do that, human?”

A shiver wracked your body, his unexpected heavy-handed tone sending a tingle straight to your groin. “S-sorry.”

Mouth etching into a sharp grin, his white eyelights flickered as he loomed over you. “heh, good girl.”

His smooth finger bones trailed up your abdomen and grasped at the clasp of your bra, fiddling with it. And all you managed to do was lay there and quiver, breaths escaping you in uneven pants as he peeled off the remainder of your clothes.

Taking a moment to lean back and tug his coat off, his bony hands found your body again, groping at your thighs before trailing up your sides and squeezing your breasts. His dull claws grazed your skin ever so lightly and each movement made your mind grow fuzzier, unable to comprehend that this was _actually happening._

Were you dreaming…?

Sans flicked one of your nipples before pinching it and rolling it around in his fingers, dragging a squeak from you. “yeah… you _really_ like monsters, don’tcha?”

He snickered, kneading your bare thigh with his other hand. “actually have a bit of a thing for humans myself,” he rumbled, “just _love_ how soft and _squishy_ you are.”

Cheeks flushing, you found yourself unable to find the words to respond as you gripped desperately onto his radius and ulna, a jolt of pleasure rocketing through you when he leaned down and twirled his tongue around your nipple. _Hahah… it’s… it’s blue…_

Trailing his nasal cavity down your abdomen, he drew lower… and lower… until you could feel his hot breath tickle your slit, forcing a shiver to roll up your spine. He snickered, pressing his mouth teasingly against you in a way that made you moan… before pulling away.

You whined at the sudden loss of contact, wanting to grab his skull and drag him back to you… only to yelp when he latched onto your hair and stuffed your panties into your mouth.

“better not touch em’ unless i say so…” His grin turned wicked and he pressed his teeth against your ear, whispering, “you got _neighbors_ … wouldn’t want them to hear, would we? that’d be just _embarrassing_ for a ‘monster-hater’ like you.”

_Oh... Oh!_

Licking the shell of your ear, he nipped your earlobe before again trailing down your body, positioning himself between your thighs upon spreading your legs. He winked at you, before taking your breath away by stroking his tongue up your slit.

You let out a stuttered moan through your gag, hands gripping at the sheets as your body simultaneously tensed and trembled. And Sans didn’t let up, mercilessly assaulting your sensitive nub with his tongue and teasingly poking at your entrance. His magic tingled you, forcing waves of pleasure up your spine that pooled in your gut. Unable to stop yourself, you clamped your thighs around his skull, only for him to pry them back apart.

_He’s-he’s strong!_

You certainly didn’t expect that from someone made of bones.

It wasn’t long before he had you nearly shrieking. You weren’t prepared for just how _long_ his tongue was as it coiled deep inside of you, nor the way his magic buzzed against your too-sensitive clit. But he kept at it… teasing you and teasing you… forcing you to suck in a breath as that pressure built in your core… until your mind blanked out when you were tipped over the edge.

You heard him chuckle when your entire body spasmed, entrance quivering, teeth clamping onto the underwear in your mouth and head spinning as climax hit you like a ton of bricks.

But just as quickly as it hit you, it was over… leaving you panting and exhausted, body shivering as sweat dripped down your forehead.

Smooth skeletal fingers tracing up your sides, Sans returned to looming overtop you, a wry grin planted on his face as he stared down at you through constricted eye lights. Flicking teasingly at your nipple in a way that made you clutch onto him, he spoke in a low whisper, “want more, human?”

All you could do was shiver, an action that made his grin stretch wider.

“hehe, that’s not good enough… nod if you want more.”

You nodded, prompting him to hum a pleased noise as he leaned back and fiddled with his shorts. You craned your head to look at him, watching as a cock the same shade and texture as his tongue was pulled out.

Snatching your wrist, he wrapped your hand around his cock and began using it to pump himself, eventually letting go and allowing you to continue the motion on your own. It felt so slick and _warm_ in your palm, thick enough where you almost couldn't wrap your hand all the way around it.

You looked to him for any indication that you were doing a good job, but he simply tilted his head and flashed you an innocent smile. “what? can’t expect me to do all the work here.”

Well... you didn’t mind at all. You’d been fantasizing about this moment for _years._

His hands danced along your skin as you stroked him, him groping and squeezing at various parts of your body as though he was obsessed with how squishy you were. You rubbed a thumb over his cockhead, prompting him to suck in a sudden intake of air as his bones rattled against you.

Pulling your hand off of him, he grabbed you by the hips and flipped you onto your stomach, situating himself behind you. A shudder wracked your body when you felt his cock trail along your backside, leaking hot precum onto your skin as he positioned himself at your entrance.

And you gasped when you felt the head of his cock begin to enter you…

... Only to whimper when he pulled it away and instead lazily stroked it against your slit.

Grabbing hold of your hair, he craned your head back and pulled the underwear from your mouth. His heated breaths wafted over your neck as his mouth hovered centimeters from your ear, a low growl escaping him.

“beg for it, human,” he rumbled against you, “tell me how badly you wanna be fucked by a monster.”

You yelped when you felt him brush his cock against your engorged clit, a heated blush rising to your cheeks when you realized just how loud you were without the gag. Body trembling, you shifted your tongue around your mouth, trying to get it working properly again.

But you didn’t even have a chance to open your mouth before he again startled you by pressing his cock against your entrance… before pulling it away once more.

“A-ah! That’s not fair!” you cried out as he continued to tease you, shooting a jolt up your spine.

He hummed, clicking his tongue. “not hearin’ begging.”

Panting heavily, you offered very little hesitation.

“Please! Please fuck me Sans! I want to be fucked by a monster!”

And you weren’t prepared for just how fast he hilted himself inside of you, forcing a surprised gasp from your lips as he stretched you. You gripped at the blankets and quite nearly bit down on them as well when he began to rut mercilessly into you.

“he…heh…” he huffed out between thrusts, hardened ribcage pressed against your back as one hand trailed south to grip your hips while the other remained tangled into your hair. “that was _prettyyy_ loud, human… someone must’ve heard that.”

Your face burned, but you were far too caught up in your own pleasure to be bothered much… ragged pants and desperate moans spilling incoherently from your mouth.

You squeaked when he yanked your head back further to stroke his tongue up your neck before nibbling at your ear, sending a shiver across your overstimulated body. He bucked into you harshly, causing your vision to momentarily blank out. And you nearly shrieked when he trailed his nasal cavity along your nape and bit down onto your shoulder.

Although it was not hard enough to draw blood, you knew in the back of your mind that this should have hurt… but in your haze it only intensified your arousal, a dribble of drool escaping from your parted lips onto the bed.

Sans let go, laving his tongue over the indents his teeth created on your skin before pressing his mouth against the side of your head.

“bet i can make ya scream even louder… let the whole world know what a dirty little human you are,” his husky voice slithered into your ear in a low whisper, sending a rush straight to your clit.

The hand on your hip tauntingly trailed inward, crawling over your pubic mound before pressing against your clit. And you did indeed scream when he rolled his fingers directly over it, dragging a mocking chuckle from him.

Untangling his other hand from your hair, he swiftly smothered your mouth. And you could practically feel his smirk form as he nuzzled his face into the crook of your neck.

He began to lazily rub circles around your clit, all the while continuing to buck into your quivering entrance. He grunted, his ragged pants hot against your skin as he worked tirelessly, vaguely blue sweat beading on his skull. His bones began to rattle, and you wondered if that meant he was getting close.

You felt yourself growing close again as well, your vision mottling with specks of light and muscles tensing as you held onto the sheets for dear life. The fire burning in your gut was growing near unbearable and had it not been for the skeletal hand holding your jaw shut, you’re certain you would be pissing off the whole building with your shrieking by now.

That pressure in your core winding up, you tensed, holding your breath… until a bout of blissful euphoria hit you that sent your body into a series of uncontrollable spasms. In your haze, you just vaguely heard a guttural grunt escape Sans as your walls twitched around his cock.

“fuck… fuck… just a little more… fuck…” he groaned out between uneven pants.

He gripped your hips and thrust into you _hard_ , pelvis smacking against your posterior and forcing a muffled yelp out of you as it he continued to stimulate your exhausted, overstimulated body. You felt his cock quiver, bones audibly shuddering, before a tingling warmth filled your abdomen.

Rolling his hips into you a few more times, he exhaled harshly and pulled out, forcing a whining moan out of you as you were left panting and utterly spent on the mattress.

Taking a brief moment to right his shorts, he crashed next to you on the bed; the only sounds filling the air being the ragged pants from the both of you.

You didn’t keep track of how long you and him laid like this. Still in your dazed high, you barely even registered when he combed his fingers through your hair.

“not dead, are you?”

Wiping the sweat from your forehead, you sat up to look at him, a heated blush rising to your face as you comprehended what you and him had just done.

His bones vibrated with laughter, making your cheeks burn even hotter.

_Say something! Say something!_

“I-I don’t… I don’t… actually _hate_ m-mo-”

But he cut you off, smacking his hand over your mouth.

Flashing you an innocent grin, he spoke, “i think we should start over… i’m sans. sans the skeleton.”

His hand left your face and he held it out to you as if to initiate a handshake.

You stared at him incredulously for a moment, brows furrowing. It seemed ridiculous to consider what he was insinuating when you had his magical semen dripping from your entrance and the scent of your arousal wafting through the air… but… you had been fantasizing about a second chance with him for months now.

In fact, it was nothing short of a dream come true.

Without another thought, you took it.


End file.
